Avocado tree called Barr Duke

ABSTRACT

A new and distinct rootstock variety of avocado tree characterized by its high yield resistance to Phytophthora root rot caused by Phytophthora cinnamomi. This variety has a high level of resistance comparable to the `Duke 7` rootstock variety. It is also characterized under some field conditions by producing a somewhat smaller tree, when grafted with a `Haas` scion, than that typical of a `Duke 7` rootstock.

DESCRIPTION

This invention relates to a new and distinct rootstock variety ofavocado tree (Persea americana var. drymifolia) designated `Barr Duke`.

`Barr Duke` is a Mexican avocado rootstock. It is a seedling of `Duke 6`and has been tested at the University of California, Riverside and atSouth Coast Field Station, Irvine, Calif. for resistance to Phytophthoraroot rot caused by Phytophthora cinnamomi. It has also been tested forroot rot resistance in various avocado grooves in southern California.`Barr Duke` was initially tested in a nutrient solution tank whichsurvived the severe root rot test and was then transplanted to infestedsoil in the greenhouse. The selection showed promise as a rootstock forroot rot resistance and additional materials of the selection werepropagated asexually for further greenhouse tests and for resistancetests in field plots. Over a period of seven years `Barr Duke` hasdemonstrated excellent growth in diseased sites. Further asexualreproduction of `Barr Duke` demonstrates that the variety remainsconsistent in the characteristics that distinguish it.

In the drawing:

FIG. 1 shows the general shape and color of the `Barr Duke` selection.

FIG. 2 shows the foliage and flowers in more detail.

FIG. 3 shows the mature green fruit of the `Barr Duke` variety.

`Barr Duke` has three primary characteristics that distinguish it. Ithas high field resistance to Phytophthora root rot. It is a Mexicanvariety and is well-adapted to California conditions. When used as arootstock, under some growing conditions, it reduces the size of theHass scion variety.

Resistance to Phytophthora cinnamomi has been tested in many experimentswith `Barr Duke` in the field. `Barr Duke` has consistently proved to beresistant, at a level comparable to the `Duke7` rootstock. In anextremely severe test where the trees were deliberately inoculated withPhytophthora at planting time, `Barr Duke` was evaluated as producinggrowth comparable to that with `Barr Duke` not inoculated withPhytophthora.

It is a less vigorous grower than `Duke 7`, `G755C` or `Thomas`, with orwithout Phytophthora root rot being present.

TREES AND FOLIAGE

`Barr Duke`, as is the case with the majority of avocado varieties, doesnot have distinctive foliage. The leaves have a distinct anise scentwhen crushed, which typifies the Mexican horticultural race.

FRUIT

Ovate; about 150 grams; smooth skin. Flesh about like `Zutano`. Withreference to the Munsell Color Charts typically `Barr Duke` has a skincolor (strong yellow green, Munsell Hue 2.5 GY 6/8) similar to thevariety `Duke`. The season is about August through September in SouthernCalifornia.

The following chart represents visual ratings of the `Barr Duke` varietycompared with other root rot resistant varieties according toobservations made at the South Coast Field Station on Aug. 14, 1986.

    ______________________________________                                                   #        #                                                                    PLANT-   SYMP-    %        AVE.                                    SELECTION  ED       TOMS     HEALTHY  STAGE                                   ______________________________________                                        BARR DUKE  10       1        90       0.2                                     G755A      10       1        90       0.4                                     THOMAS     10       1        90       0.6                                     G755C      10       3        70       1.4                                     G6 PARENT  10       4        60       1.4                                     TORO CANYON                                                                              10       5        50       2.4                                     D9         10       5        50       2.8                                     DUKE 7     10       6        40       2.2                                     G755B      10       6        40       2.4                                     G1033      10       8        20       4.3                                     TOPA-TOPA  10       8        20       6.1                                     BORCHARD   10       9        10       7.4                                     ______________________________________                                    

We claim:
 1. The new and distinct rootstock variety of avocado treeherein described and illustrated and identified by the characteristicsenumerated above.